What causes seismic heterogeneities outside the LLSVPs in the lowermost mantle?

Abstract:

The Earth’s lowermost mantle is characterized by two large-low shear velocity provinces (LLSVPs). Surrounding the LLSVPs are regions with generally higher-than-average seismic velocities. Interestingly, seismic observations have also revealed wide-spreading heterogeneities in regions outside the LLSVPs, but their origin remain unclear. Here, through 3D thermal and thermochemical numerical modeling experiments, I show that linear, ridge-like hot thermal anomalies, or thermal ridges, form in the relatively cold, downwelling regions of the lowermost mantle (Li, 2020). Like the sub-lithosphere small-scale convection (SSC), the thermal ridges form as a result of SSC from the basal thermal boundary layer and they extend in directions parallel to the surrounding mantle flow. The formation of thermal ridges in subduction regions of the lowermost mantle is very sensitive to the thermal structures of the subducted materials, and thermal heterogeneities brought to the bottom of mantle by subducting slabs greatly promote the formation of thermal ridges.

Based on this study (Li, 2020), I further perform high-resolution 3D models with Earth-like single-sided subduction, and I find that the segregation, advection and distribution of subducted oceanic crust in the subduction region of the lowermost mantle are greatly controlled by the structure and morphology of the subducted slabs. Therefore, subducted slabs contribute to thermal and chemical heterogeneities in the lowermost mantle that may cause seismic heterogeneities outside the LLSVPs.

Reference:

Li, M. (2020), The Formation of Hot Thermal Anomalies in Cold Subduction‐Influenced Regions of Earth's Lowermost Mantle, Journal of Geophysical Research: Solid Earth, 125(6), e2019JB019312, doi:10.1029/2019jb019312.